I don't see why that wouldn't work. But, as you're in Bolivia, make sure you get the right genus of sweet potatoes--you want a species of ipomea, not a dioscorea. Mexican or south american yams tend to be of the dioscorea genus, which is totally unrelated, and I have no idea if dioscorea yams have the same kind of amylase content (or any at all). You could also try bananas, which apparently have amylase in them as well.
What I'd recommend, if you don't want to include the potatoes in the mash, is do a few mashes with the same potatoes to get as much amylase out of them as you can, and then try to concentrate the liquid via low-temperature evaporation. Don't heat above 160 or you risk denaturing the enzymes.
Doing this, I wouldn't advise you to expect comparable efficiency to malted barley, just FYI. It'll be a good bit lower. You're really better off just malting your own barley, TBH. But if you can get sweet potatoes on the super-cheap, no reason not to give it a go!